Thermally controllable microhotplate sensors for gases and vapors are known in the art. Examples of such microhotplate sensors are disclosed in U.S. Pat. Nos. 5,464,966 and 5,345,213. Such sensors typically comprise a support substrate, a heating element thermally isolated from the substrate, a conductive heat distribution plate formed above the heating element, and a layer of a chemically active material formed above the heat distribution plate. The sensors are characterized by having a low mass (on the order of about 0.2 .mu.g) such that the temperature of the sensor can be controlled over a wide range (on the order of 20.degree. C. to about 1200.degree. C.) and changed very rapidly (with rise times on the order of about 1 msec). These sensors can be formed into an array, the individual sensors constituting "pixels" or elements in the sensor array.
Further details concerning the function and operation of the foregoing sensors can be found in the aforementioned U.S. Pat. Nos. 5,345,213 and 5,464,966 and in Cavicchi et al., "Fast Temperature Programmed Sensing for Micro-Hotplate Gas Sensors," IEEE Electron Device Letters 16(3):1-3 (1995) and in Suehle et al., "Tin Oxide Gas Sensor Fabricated Using CMOS Micro-Hotplates and in-situ processing," IEEE Electron Device Letters 14(3):118-120 (1993). For present purposes, it is sufficient to state that the chemically active material in such sensors interacts with a detected analyte, thus causing a change in the conductance of the chemically active material. The change in conductance of the chemically active material thus may be used to detect the presence of the analyte.
While such sensors are useful in detecting the presence or absence of a single analyte, it can be more difficult for the sensor to differentiate between two or more analytes, especially when the analytes are chemically similar. For example, when the sensor is operated with a linear temperature profile that is pulsed, i.e., cyclically varied, the conductance output profile over time of the sensor for a first analyte (for example, methanol) often is very similar to that for a second, chemically similar analyte (for example, ethanol). When the sensor is introduced to an unknown sample containing one or both of the first and second analytes, it thus can be difficult for the operator to differentiate between the first and the second analytes based on the sensor output profile obtained.
It is a general object of the invention to provide a method and apparatus for operating a sensor, such as a microhotplate and apparatus, to enhance the difference between the output profiles of the sensor when introduced to first and second analytes to thereby assist an operator in differentiating between first and second analytes in a sample of unknown composition. A further general object is to provide a method and apparatus for operating a sensor in which the detection time is minimized while still enhancing the difference in the sensor output profiles as between the first and second analytes.